The Blue Dogs went extinct because Barack Obama was so effective at pulling his congregation –err, constituency to the extreme Left. Anyone whose views differed from mindlessly “progressive” was the enemy. Add to the fact that the Blue Dogs have traditionally been a rather ethical group, versus the “ends justify the means” theme of modern Democrat politics.
But in light of what the 2016 election exposed, there’s a growing group of thinking, reasonable leftists/progressives who are furious with the abject corruption of the DNC and the coordinated attacks on Donald Trump. It’s difficult to gauge how large this group is so it may be early for the Blue Dogs. But they could become a significant force again in 2020.